Knowledge Organiser: Physical Geography Fieldwork (Rivers)
Bradshaw Model Predictions
- Discharge: increases (more tributaries)
- Velocity: increases (less friction, wider channel)
- Width and depth: both increase (erosion)
- Pebble size: decreases (attrition)
- Pebble roundness: increases (attrition)
- Gradient: decreases (erosion to base level)
Data Collection Methods
- Velocity: float method (orange over 10 m) or flow meter
- Width: tape measure from bank to bank
- Depth: metre ruler at regular intervals across channel
- Pebble size: ruler, long axis in mm, 10–20 random pebbles
- Pebble roundness: Powers Roundness Scale (1–6)
- Gradient: clinometer and ranging poles
- Repeat each measurement 3× and take the mean
Sampling and Risk
- Sites: systematic (every 100–200 m downstream), 5–8 sites minimum
- Pebbles: random (reach in without looking)
- Key risks: slipping; deep/fast water; flash floods
- Control measures: non-slip footwear; buddy system; weather checks; never enter water above knee
- Spearman's rank: tests if correlation is statistically significant; critical value ±0.536 (n=14)
Evaluation Points
- Float only measures surface velocity (overestimates mean) — improve with flow meter
- One visit may not be typical — improve with repeat visits in different seasons
- Small pebble sample (10 per site) — improve by increasing to 20+
- Subjective roundness (Powers Scale) — improve by using one assessor throughout
- Too few sites (5) — improve by adding sites especially in middle course
- Reliability: consistency; Validity: measuring what you intend
Common Mistakes
- Not linking measurements to the Bradshaw Model: Every measurement must connect to a Bradshaw Model prediction — don't just record channel width; explain that you expect it to increase downstream as lateral erosion widens the channel
- Using the float method without acknowledging its limitation: The float measures surface velocity, which overestimates mean channel velocity — always state this limitation and suggest using a flow meter as an improvement
- Selecting pebbles by hand without randomising: Deliberately picking pebbles introduces bias — explain that you reached in without looking to ensure random selection and reduce the risk of choosing only accessible pebbles
- Weak Spearman's rank interpretation: Don't just state the r value — compare it to the critical value (±0.536 for n=14 at 95% confidence), state whether the correlation is statistically significant, and link the result back to your hypothesis
Memory Aid
- D — Discharge increases
- V — Velocity increases
- W — Width increases
- L — Load roundness increases
- G — Gradient decreases (the exception — use this to remember the odd one out)
- And: load size decreases; depth increases